Chromatic dispersion (i.e. wavelength dependence of group velocity) results in temporal spreading of optical pulses as they propagate along a signal path such as an optical fibre. This sets a limit on the maximum propagation distance, before adjacent pulses start overlapping severely and become indistinguishable. At this point, optical pulses should be recompressed to their initial duration, the recompression being done electrically or optically. Chromatic-dispersion propagation limits depend on the propagation medium (fibre type), as well as, the initial pulse duration (signal bit rate).
Chromatic dispersion can be characterised by first-, second-, third- and higher-order components of dispersion [1]. First order dispersion corresponds to the average time delay of the pulse. As the pulse propagates along, it will typically disperse, that is, pulse spreading occurs and the pulse typically increases in length. Second order dispersion corresponds to the average increase in pulse width per wavelength per unit length. Third order dispersion corresponds to the variation in pulse spreading per wavelength per unit length. At 1550 nm, the first order dispersion in a standard single mode telecommunication grade optical fibre is approximately 5×106 ps/km, the second order dispersion is 17 ps/nm/km, and the third order dispersion is 0.06 ps2/nm/km.
Tuneable dispersion compensation is important in high-speed, high performance long-haul and metro telecommunication systems. Although in the metro systems the transmission distances are much shorter than the ones in long-haul systems, it is quite likely that they will vary substantially as the system is dynamically reconfigured and certain channels are switched at the various nodes. Tuneable dispersion compensation modules (DCMs) will be used either in a static tune-and-set or in a fully dynamic tuning mode, depending on the system architecture, bit rates and transmission distances.
A number of different tunable DCMs have been proposed based on fibre Bragg gratings (FBGs). They can be divided into two broad categories. The first involves standard linearly-chirped apodized gratings, which are tuned by applying a perturbation whose strength varies linearly along the grating length. The second involves nonlinearly-chirped gratings, which are tuned by applying a perturbation whose strength is typically constant along the grating length (although it can be non-linear).
The most common techniques for applying an additional linear chirp on a FBG involve a temperature [2,3] or strain gradient [4,5] along the grating length. The main problem with this technique originates from the characteristics of the grating. Linearly chirped, apodized gratings are known to suffer from an underlying non-linearity of the group delay variation with wavelength, due to the presence of residual relatively strong, overlapping band-gaps within the reflection spectrum [6]. This group-delay non-linearity can be improved by applying a tighter apodisation profile, although this is accomplished at the expense of the reflection spectrum squareness [6]. For all apodisation profiles, the group-delay nonlinearity gets worse when the total (initial+induced) chirp and, consequently the average linear dispersion, reduce. This compromises the performance and limits the tuning range of the DCM considerably. The grating is also relatively long and difficult to manufacture.
In the second category, a DCM is accomplished by using a more complex grating and a uniform perturbation (such as uniform stretching/compressing or uniform heating/cooling). The grating is non-linearly chirped so that it exhibits both second- and third-order chromatic dispersion. Such a device exhibits a linearly varying dispersion across the reflection band. Chromatic dispersion tuning is achieved by shifting the reflection spectrum relative to optical carrier wavelength. In the simplest configuration, such a DCM can be implemented using only one non-linearly chirped grating [7]. This approach, however, inevitably introduces an amount of third-order chromatic dispersion, which can potentially limit the usefulness of the device at high bit rates (e.g. ≧40 Gb/s). In addition, any relative transmitter/DCM wavelength drift results in chromatic dispersion variation. These problems can be overcome by connecting two identical (twin) non-linearly chirped gratings (in an inverse manner) into a four-port circulator [8,9]. This configuration cancels out the third-order chromatic dispersion of the individual gratings and provides pure second-order chromatic dispersion compensation (a much desirable feature). However, the penalty to be paid, compared to the other single-grating approaches, is the increased number (twice as many) gratings and the use of one four-port or two three-port circulators per DCM unit. In addition, the gratings are relatively long and difficult to manufacture.
There are a variety of other DCM technologies including dispersion compensating fibres and various filter and device types such as concatenated Mach Zehnder interferometers, ring interferometers, and arrayed waveguide gratings. Several of these technologies provide lower cost solutions, but at the expense of reduced performance, particularly in tuneable configurations.
An aim of the present invention is to produce an apparatus for dispersion compensating a signal that propagates along a signal path that reduces the above aforementioned problems.